Wire the fourth mandated §23 metric to a real producer. The histogram
spans two reconcile passes, so anchor and observation must persist in
status:
- Add status.startRequestedAt, set once on the first Starting reconcile
of a start attempt and cleared on Stopped so the next start re-anchors.
- Observe felis_start_duration_seconds exactly when readiness is first
reached (ReadySignalAt - StartRequestedAt), guarded so a server that
reaches ready without a Starting pass records nothing.
- Mirror the field into the deepcopy and the structural CRD schema so the
apiserver does not prune it on patchStatus round-trips.
- Promote prometheus/client_golang and client_model to direct deps now
that the operator and its tests import them.
Tests drive a step clock through Starting -> Running asserting the exact
observed duration, and through Running -> Stopped asserting the metric is
observed once and the anchor clears.
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